Inert Wear Resistant PTFE-Based Solid Lubricant Nanocomposite

ABSTRACT

A PTFE-based composite material includes a PTFE major phase filled with a metal oxide minor phase. The major phase is intermixed with the metal oxide minor phase, wherein the minor phase includes a plurality of irregularly shaped metal oxide nanoparticles. The irregularly shaped nanoparticles provide substantial reductions in steady state wear rate over otherwise similar nanocomposites. The metal oxide can comprise aluminum oxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No.60/685,275 entitled “INERT WEAR RESISTANT PTFE-BASED SOLID LUBRICANTNANOCOMPOSITE” filed on May 27, 2005, the entirety of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government may have certain rights to this inventionpursuant to NSF Grant No. CMS-0219889 and AFOSR-MURI Grant No.FA9550-04-1-0367.

FIELD OF THE INVENTION

The invention relates to inert PTFE-based low wear composite materials.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) exhibits desirable tribologicalcharacteristics, including low friction, high melting temperature andchemical inertness. Based on these characteristics, PTFE is a frequentlyused solid lubricant both as a filler and matrix material. Without afiller, however, PTFE suffers from a relatively high wear rate,generally precluding its use in frictional applications, including useas a bearing material.

As a matrix material, PTFE has been successfully filled with variousnanoparticles, including alumina, zinca, and carbon nanotubes. Regardingalumina filling, Sawyer et al. [Sawyer, W. G., Freudenburg, K. D.,Bhimaraj, P., and Schadler, L. S., (2003), “A Study on the Friction andWear of Ptfe Filled with Alumina Nanoparticles,” Wear, 254, pp. 573-580]discloses 38 nm substantially spherical shaped Al₂O₃ filler particlesfor improving the wear performance of PTFE. The wear resistance of thisnanocomposite was reported to increase monotonically with filler wt %,eventually being 600 times more wear resistant than unfilled PTFE at aloading of 20 wt. % Al₂O₃. Although the wear performance provided byPTFE/alumina nanocomposites disclosed by Sawyer et al. represents amajor improvement over PTFE, the high filler percentage required toreach the desired wear level significantly raises the cost of thenanocomposite. In addition, for certain applications wear rates lowerthan 600 times better than PTFE are desirable and may even be required.Accordingly, a PTFE nanocomposites is needed which provides improvedwear resistance, while at the same time requiring a lower fillerpercentage as compared to the PTFE nanocomposites disclosed by Sawyer etal.

SUMMARY

A PTFE-based composite material comprises a PTFE comprising major phasefilled with a metal oxide minor phase. The major phase is intermixedwith the metal oxide minor phase, wherein the minor phase comprises aplurality of irregularly shaped metal oxide nanoparticles. The minorphase can comprise 1 to 10 wt. % of said composite, such as 3 to 7 wt.%. In one embodiment, the metal oxide nanoparticles have shapescharacteristic of milled particles. The metal oxide can comprisealuminum oxide. Regarding performance, the composite can provide asteady state wear rate of K<2×10⁻⁵ mm³/(Nm) for a 5% wt. % minor phase.composite.

A method of forming wear resistant composite materials comprises thesteps of blending nanoscale metal oxide particles and PTFE particles,wherein the metal oxide particles are irregular shaped nanoparticles,and heating the nanoscale metal oxide particles and PTFE particles toform a nanocomposite. The heating step can comprise compression molding.A jet milling apparatus is preferably used for the blending step. In oneembodiment, the metal oxide comprises aluminum oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features andbenefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

FIG. 1 provides surface profilometry data from PTFE nanocompositesaccording to the invention obtained from a scanning white-lightinterferometer with a 20× objective; clockwise from top left:electro-polished, lapped, dry-sanded and wet-sanded surfaces.

FIG. 2 shows a schematic of the tribometer used for friction and weartesting of PTFE nanocomposites according to the invention described inthe Examples provided herein.

FIG. 3 provides average friction coefficient data for all 44 nmPTFE/alumina nanocomposites according to the invention plotted versusweight percent of alumina filler particles. The normal load was 250N andthe sliding speed was 50 mm/s (the sliding distances varied). The errorbars represent the standard deviation of the friction coefficientmeasured during each test.

FIG. 4 provides average wear rate data for all 44 nm PTFE nanocompositesaccording to the invention plotted versus weight percent of aluminafiller particles. The normal load was 250N and the sliding speed was 50mm/s (the sliding distances varied). The error bars represent thestandard uncertainty of the measurements.

FIG. 5 provides average wear rate data for 44 nm PTFE nanocompositesaccording to the invention plotted versus the RMS roughness of thecounterfaces and Rq/Df. The normal load was 250N and the sliding speedwas 50 mm/s (the sliding distances varied). The vertical error barsrepresent the standard uncertainty of the measurement and the horizontalerror bars represent the standard deviation of the RMS roughness over 5samples.

FIG. 6 provides average friction coefficient data for PTFEnanocomposites according to the invention plotted versus the RMSroughness of the counterfaces. The normal load was 250N and the slidingspeed was 50 mm/s (the sliding distances varied). The vertical errorbars represent the standard deviation of the measurements and thehorizontal error bars represent the standard deviation of the RMSroughness over 5 samples.

FIG. 7 provides average wear rate data for PTFE nanocomposites accordingto the invention plotted versus the RMS roughness of the counterfaces.The normal load was 250N and the sliding speed was 50 mm/s (the slidingdistances varied). The vertical error bars represent the standarduncertainty of the measurement and the horizontal error bars representthe standard deviation of the RMS roughness over 5 samples.

FIG. 8 provides scanning white-light interferometry images ofrepresentative transfer films according to the invention; a) 5 wt % 44nm PTFE nanocomposites according to the invention on the lappedcounterface, b) 5 wt. % 44 nm PTFE nanocomposites according to theinvention on the dry-sanded counterface, c) 5 wt. % 80 nm PTFEnanocomposites according to the invention on the lapped counterface, d)5 wt. % 80 nm PTFE nanocomposites according to the invention on thewet-sanded counterface, e) 5 wt % 500 nm PTFE nanocomposites accordingto the invention on the lapped counterface, f) 5 wt. % 500 nm PTFEnanocomposites according to the invention on the dry-sanded counterface

FIG. 9 provides wear-rate and coefficient of friction data for PTFEnanocomposites according to the invention plotted versus transfer filmthickness. There is a correlation seen between wear and transfer filmthickness.

DETAILED DESCRIPTION

A polytetrafluoroethylene (PTFE)-based composite comprises a PTFE majorphase filled with a metal oxide minor phase. The major phase isintermixed by the metal oxide minor phase. The metal oxide minor phasecomprises a plurality of irregularly shaped metal oxide nanoparticles.The metal oxide can be a variety of metals oxides, including, but notlimited to, aluminum oxide (e.g. alumina), zinc oxide, zirconium oxideand titanium dioxide. The metal oxide particles preferably generallycomprise from 1 to 10 wt. % of the composite, such as 1, 2, 3, 4, 5, 6,7, 8, 9 or 10% of the composite, but can be higher % (e.g. 15 wt. %) orlower % (0.5 wt. %) as compared to this range. Composites according tothe invention provide higher wear resistance at much lower nanoparticleloading than current nanocomposite technology.

As used herein, the term “irregular shape” refers to non-sphericalshaped particles, such as the shapes produced by crushing or millingaction. The particles of irregular shape thus have asperities, pointsand edges as well as some flat areas. Such particles are availablecommercially, such as from Nanophase Technologies Corporation,Romeoville, Ill. or Alfa-Aesar (Ward Hill, Mass.), or can be formed bymilling.

As used herein, the term “PTFE” includes polytetrafluoroethylene as wellas its derivatives, composites and copolymers thereof, wherein the bulkof the copolymer material is polytetrafluoroethylene, includingcopolymers of tetrafluoroethylene and hexafluoro(propyl vinyl ether),copolymers of tetrafluoroethylene andperfluoro-2,2-dimethyl-1,3-dioxole, and copolymers oftetrafluoroethylene and vinyl fluoride, poly(vinyl fluoride),poly(vinylidene fluoride), polychlorotrifluoroethylene, vinylfluoride/vinylidene fluoride copolymer, and vinylidenefluoride/hexafluoroethylene copolymer. Where the term “PTFE” is usedherein to describe polytetrafluoroethylene that is copolymerized withone of the above-named polymers, it is contemplated that the actualpolytetrafluoroethylene content in the copolymer is about 80% by weight,or higher.

Previous PTFE-alumina composites, such as disclosed by Sawyer et al. asnoted in the Background above, utilized substantially spherical aluminaparticles. The inventors have discovered that irregularly shaped fillernanoparticles provide a much higher wear resistance to the composite,and provide the higher wear resistance at a significantly lower fillerwt. %. For example, measured wear resistance for PTFE-aluminananocomposites at a 5 wt. % filler level according to the invention havebeen found to be about 3,000 greater than PTFE and 10 times better thanthe most wear resistant Sawyer et al. PTFE-alumina composites whichrequired 20 wt. %, or more, of alumina filler.

Nanocomposites according to the invention are highly chemically inert;derived from the highly non-reactive nature of both PTFE and the variousmetal oxides that may comprise the composite material. Very causticenvironments may necessitate the use of PTFE which wears very rapidly,making frequent replacement a necessity. The addition of metal oxideparticles in composites according to the invention will increase thewear resistance of the PTFE without sacrificing chemical inertness.Nanoparticles have the advantages of non-abrasiveness, and high numberdensity at low filler wt %.

Although not required to practice the invention, Applicants provide thefollowing proposed mechanism to explain the improved wear rateperformance of PTFE-based composites according to the invention beingobtainable at low metal oxide filler percentages. Since the metal oxideand the PTFE do not interact chemically, the superior wear resistance ofthis nanocomposite at low filler wt. % may be attributed to the inherentmechanical interaction of PTFE with the irregularly shaped metal oxidenanoparticles as opposed to the weak engagement that occurs with thespherical particles that are typically used. The irregularly shapedmetal oxide particles are believed to substantially surround the PTFEparticles, but generally allow fibrils of PTFE to connect thecompartmentalized PTFE major phase.

The invention is expected to be useful for a wide variety ofapplications whenever friction occurs and caustic chemicals are used,such as for fittings, bushings, and valves. The semiconductor industryhas processes where PTFE is currently used at great expense for etchingchemicals.

EXAMPLES

The present invention is further illustrated by the following specificExamples, which should not be construed as limiting the scope or contentof the invention in any way.

Wear and friction tests were performed on PTFE nanocomposites accordingto the invention against counterfaces of varying surface finishes. Grade304 stainless steel counterfaces, measuring 38 mm×25.4 mm×3.4 mm wereused in these wear tests. This material had a measured Rockwell Bhardness of 87 kg/mm². The surfaces were prepared using 4 differenttraditional finishing processes, including electro-polishing, lapping,wet-sanding, and dry-sanding. The electro-polished samples were preparedby wet-sanding with 600 grit silicon-carbide paper, followed by lapping,and finished by electro-polishing. Similarly, the lapped samples wereinitially wet sanded with the 600 grit silicon-carbide paper and thenlapped. The wet-sanded samples were only exposed to the 600 gritsilicon-carbide paper. The dry-sanded samples were initially wet sandedand then roughened with 80 grit “coarse” silicon-carbide paper. Thesamples were examined under a scanning white light interferometer with a20× objective. Areas of 230 μm×300 μm were measured on 5 differentsamples from each batch. A gray-scale contour plot with accompanyingline scans, amplitude parameters R_(a) (average roughness) and R_(q)(root mean squared roughness), and histograms of the surfaces are shownin FIG. 1. For these surfaces the average R_(q) roughness and standarddeviation between 5 samples (σ) was R_(q)=88 nm σ=16 nm, R_(q)=161 nmσ=35 nm, R_(q)=390 nm σ=27 nm, and R_(q)=578 nm σ=91 nm for theelectro-polished, lapped, wet-sanded, and dry-sanded surfacesrespectively. Four PTFE based nanocomposites, of composition 0, 1, 5 and10 weight percent 44 nm Al₂O₃, were processed following the sameapproach as reported in [Sawyer, W. G., Freudenburg, K. D., Bhimaraj,P., and Schadler, L. S., (2003), “A Study on the Friction and Wear ofPtfe Filled with Alumina Nanoparticles,” Wear, 254, pp. 573-580].Briefly, the procedure involves blending the appropriate masses ofconstituent powders using a jet milling apparatus. The mixtures werethen compression molded, machined, measured and weighed; a density ofthe sample was calculated from these measurements. Results andorganization of the experiments varying surface roughness and fillerloading holding particle size constant are shown in Table 1 below. Theexperimental design consists of 4 counterface conditions and 4 particleloadings with repeat tests of unfilled PTFE and 5 wt. % alumina filledPTFE on the lapped counterface. This provides some indication of scatterin the data from processing variations.

TABLE 1 Tribological results of an experimental matrix that uses a newcounterface and individually made composites samples with an aluminafiller particle size of 44 nm. counterface weight percent of 38 nmalumina filler preparation 0% 1% 5% 10% electro- K = 695 K = 174 K = 52K = 75 polished μ = 0.165 μ = 0.175 μ = 0.191 μ = 0.178 lapped

K

₆ = 586 K = 89

K

₄ = 66 K = 38

μ

₆ = 0.136 μ = 0.174

μ

₄ = 0.173 μ = 0.184 wet K = 770 K = 85 K = 99 K = 50 sanded μ = 0.135 μ= 0.172 μ = 0.162 μ = 0.163 dry sanded K = 634 K = 293 K = 294 K = 97 μ= 0.142 μ = 0.159 μ = 0.145 μ = 0.183 The 

 

_(n) represents mean values over n experiments, otherwise only a singleexperiment was run. The units on wear rate K are ×10⁻⁶ mm³/(Nm).

It was hypothesized that increases in the non-dimensional roughnessparameter R_(q)/D_(f) (the ratio of the root-mean-squared roughness tothe characteristic filler diameter) would increase the wear rate. Whilethis parameter can be easily varied by changing only the surfaceroughness, another series of experiments described here varies thefiller particle size as well. All of these composites were processedidentically. The experimental design and results varying filler particlesize and roughness holding filler loading constant are given in Table 2below.

TABLE 2 Tribological results of an experimental matrix that uses a newcounterface and individually made composites samples with an aluminafiller particle noted. counterface size of PTFE filler in 5 wt. %composites preparation unfilled 44 nm 80 nm 500 nm electro- K = 695 K =52 K = 0.80 K = 70.3 polished μ = 0.165 μ = 0.191 μ = 0.191 μ = 0.152lapped

K

₆ = 586

K

₄ = 66 K = 0.84 K = 47.4

μ

₆ = 0.136

μ

₄ = 0.173 μ = 0.158 μ = 0.174 wet K = 770 K = 99

K

₂ = 8.74 K = 64.9 sanded μ = 0.135 μ = 0.162

μ

₂ = 0.151 μ = 0.168 dry K = 634 K = 294 K = 0.95

K

₂ = 664 sanded μ = 0.142 μ = 0.145 μ = 0.141

μ

₂ = 0.145 Each composite is 5 wt. % filler, balance PTFE. The 

 

_(n) represents mean values over n experiments, otherwise only a singleexperiment was run. The units on wear rate K are ×10⁻⁶ mm³/(Nm).

The linear reciprocating tribometer to obtain wear resistance data fromPTFE nanocomposites according to the invention is shown schematically inFIG. 2. This tribometer is described in much more detail in [[Sawyer, W.G., Freudenburg, K. D., Bhimaraj, P., and Schadler, ES., (2003), “AStudy on the Friction and Wear of Ptfe Filled with AluminaNanoparticles,” Wear, 254, pp. 573-580]. Prior to testing, thecounterfaces were washed in soap and water, cleaned with acetone,sonicated for ˜15 minutes in methanol, and then dried. Thenanocomposites were wiped down with methanol but were not washed orsonicated. A normal force of 250 N was continuously monitored andcontrolled. The normal load, friction force, stroke and pin displacementwere continuously measured and recorded using a data acquisition system.The reciprocating length was 25.4 mm with an average sliding speed of50.8 mm/s. The total distance of sliding depended on the wear resistanceof the sample but, in general, was on the order of 500 m. The entireapparatus is located inside a soft-walled clean room with conditionedlaboratory air of relative humidity between 25-50%.

Varying Filler Loading and Surface Finish at Constant Filler Diameter

The average friction coefficient data for these experiments are plottedin FIG. 3 versus filler weight percent. The error-bar on a coefficientof friction datum is the standard deviation of the friction coefficientdata collected during the entire test. The friction coefficient was notsignificantly affected by changes in composition or surface roughness.It has the general trend of increasing slightly with increasing fillerconcentration and decreasing surface roughness. Friction is thought toincrease with the addition of the ceramic particles because the fillerand counterface is a higher friction pair than the PTFE and counterface.

The wear rates and uncertainties for these experiments were calculatedusing single point measurement of mass loss at the conclusion of thetest. Numerous interrupted experiments were conducted to support thereasonableness of this method. The interrupted measurements showed alinear trend of volume lost with sliding distance. These data areplotted in FIG. 4 versus filler weight percent. The error-bar on a wearrate datum is the standard uncertainty of this measurement. Thewear-rate was found to decrease monotonically with increasing fillercontent. FIG. 5 is a graph of the wear rate data plotted versusdimensionless roughness R_(q)/D_(f). There is no relationship betweenwear-rate and normalized counterface roughness, but the wear-rateappears to be minimized on the lapped counterface (R_(q)/D_(f)˜4). Thisapparent optimum may be due to filler particles mechanically engagingthe valleys of the negatively skewed lapped counterface. Scanningwhite-light interferometric examination of the counterfaces revealed atrend of decreasing debris size, and thinner, more uniform transferfilms with increasing filler loading and decreasing wear-rates.

Filler accumulation at the sliding interface should result in awear-rate that decreases with increasing sliding distance. The wearvolume of this particular nanocomposite is a linear function of theproduct of normal load and sliding distance, which suggests that filleraccumulation at the interface is not the wear reduction mechanism. Thelinear trend of this material suggests that the surface composition isnear steady-state at the onset of sliding.

Varying Filler Size and Surface Finish at Constant Filler Loading

The average friction coefficient data with error bars calculated asdiscussed previously are plotted versus counterface R_(q) in FIG. 6, andgiven in Table 2 above. No conclusive trends in the frictioncoefficients of the 44 nm, 80 nm, and 500 nm composites were observed.Steady-state wear rates were calculated from interrupted massmeasurements because the 80 nm composites exhibited a transient regionof substantially higher wear-rate. This method of calculating wear-rateand uncertainty is described in detail by Schmitz et al [Schmitz, T. L.,Action, J. E., Burris, D. L., Ziegert, J. C., and Sawyer, W. G., (2004),“Wear Rate Uncertainty Analysis,” accepted for publication ASME Journalof Tribology.].

The results of these wear tests are plotted in FIG. 7 versus thecounterface R_(q). In all cases the wear rate was the lowest on thelapped counterface, lapped counterfaces being a common and inexpensivefinishing technique. The wear of unfilled PTFE is relatively insensitiveto counterface roughness, varying by the scatter in the repeatexperiments. The most prominent feature in this graph is the 100×reduction in wear rate for the 80 nm composite over the othercomposites. This may be the result of filler accumulation at the slidinginterface by preferential removal of PTFE since this was also the onlycomposite to have transient wear characteristics. However, wear rate forthis composite increased by an order-of-magnitude on the wet-sandedcounterface. The 44 nm and 500 nm composites had increased wear-rates onthe dry-sanded counterface.

Transfer Film Examination

Various surface parameters are known significantly effect tribologicalinteractions. In general, surfaces with negatively skewed histogramshave good tribological properties. These surfaces are thought to performwell because the asperity peaks are broad and less abrasive while thevalleys are deep and sharp providing sites for transfer film engagement.Perhaps the most important result is that the composites did not show asstrong a sensitivity to the counterface roughness as was postulated. Infact, these nanocomposites can be used quite effectively on engineeringsurfaces and their ability to provide reduced wear rates is not limitedto operation on the traditionally highly finished surfaces used in mostlaboratory testing.

One constant in filled PTFE composites research is that low wear-ratesare accompanied by small wear debris. The nature of the transfer films,and specifically the thickness and coverage was quantitatively analyzedusing a scanning white-light interferometer. The worst performing andbest performing transfer films for each composite are shown in FIG. 8.It is evident from these scans that there is a direct relationshipbetween the thickness of the transfer film and the wear-rate.Additionally, the thin transfer films appear more uniformly distributedacross the counterface, while the thicker films are more banded in thedirection of sliding for a given composite. FIG. 9 shows wear-rate andfriction coefficient plotted versus maximum transfer film thickness. Thewear-rate data follow a power law curve fit. The friction coefficientappears to be independent of transfer film thickness and morphology.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as described in theclaims.

1. A PTFE-based composite material, comprising: a PTFE comprising majorphase filled with a metal oxide minor phase, wherein said major phase isintermixed with said metal oxide minor phase, said minor phasecomprising a plurality of irregularly shaped metal oxide nanoparticles.2. The composite of claim 1, wherein said minor phase comprises 1 to 10wt. % of said composite.
 3. The composite of claim 2, wherein said minorphase comprises 3 to 7 wt. % of said composite.
 4. The composite ofclaim 2, wherein said metal oxide nanoparticles have shapescharacteristic of milled particles.
 5. The composite of claim 1, whereinsaid metal oxide comprises aluminum oxide.
 6. The composite of claim 1,wherein said composite provides a steady state wear rate of K<2×10⁻⁵mm³/(Nm) for a 5% wt. % minor phase composite.
 7. A method of formingwear resistant composite materials, comprising the steps of: blendingnanoscale metal oxide particles and PTFE particles, wherein said metaloxide particles are irregular shaped nanoparticles, and heating saidnanoscale metal oxide particles and PTFE particles to form ananocomposite.
 8. The method of claim 7, wherein said heating stepcomprises compression molding.
 9. The method of claim 7, wherein said ajet milling apparatus is used for said blending step.
 10. The method ofclaim 7, wherein said metal oxide comprises aluminum oxide.